1. 18.3 Building the Tree of Life

2. THINK ABOUT IT
The process of identifying and naming all known organisms, both living and extinct, is a huge first step toward the goal of systematics.
The real challenge, however, is to group everything—from bacteria to dinosaurs to blue whales—in a way that reflects their evolutionary relationships.
Over the years, new information and new ways of studying organisms have produced major changes in Linnaeus’s original scheme for organizing living things.

3. Changing Ideas About Kingdoms
During Linnaeus’s time, living things were classified as either animals or as plants.
As biologists learned more about the natural world, they realized that Linnaeus’s two kingdoms—Animalia and Plantae—did not reflect the full diversity of life.
Classification systems have changed dramatically since Linnaeus’s time, and hypotheses about relationships among organisms are still changing today as new data are gathered.

4. Five Kingdoms
At first, all microorganisms were placed in their own kingdom, named Protista.
Later, yeasts and molds, along with mushrooms, were placed in their own kingdom, Fungi.
Later still, scientists realized that bacteria lack the nuclei, mitochondria, and chloroplasts found in other microorganisms. All prokaryotes (bacteria) were placed in yet another new kingdom, Monera.
Single-celled eukaryotic organisms remained in the kingdom Protista.

5. Changing Ideas About Kingdoms

6. Six Kingdoms
By the 1990s, researchers had learned that the organisms in kingdom Monera were actually two genetically and biochemically different groups.
The monerans were placed in two kingdoms—Eubacteria and Archaebacteria. There are now six kingdoms.

7. Three Domains
Genetic analysis has revealed that the two main prokaryotic kingdoms are more different from each other, and from eukaryotes, than previously thought. So, biologists established a new taxonomic category—the domain. A domain is a larger, more inclusive category than a kingdom.
Under this system, there are three domains—domain Bacteria (corresponding to kingdom Eubacteria), domain Archaea (corresponding to kingdom Archaebacteria), and domain Eukarya (corresponding to kingdoms Fungi, Plantae, Animalia, and kingdom “Protista”).

9. The Tree of All Life
Modern evolutionary classification is a rapidly changing science with the difficult goal of presenting all life on a single evolutionary tree.
The tree of life shows current hypotheses regarding evolutionary relationships among the taxa within the three domains.

10. The Tree of All Life

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Domain Bacteria
Members of the domain Bacteria are unicellular prokaryotes.
Their cells have thick, rigid cell walls that surround a cell membrane. Their cell walls contain peptidoglycan.
Some photosynthesize, while others do not. Some need oxygen to survive, while others are killed by oxygen.
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The domain Bacteria corresponds to the kingdom Eubacteria.
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Domain Archaea
Members of the domain Archaea are unicellular prokaryotes.
They live in extreme environments. Many of these bacteria can survive only in the absence of oxygen.
Their cell walls lack peptidoglycan, and their cell membranes contain unusual lipids not found in any other organism.
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The domain Archaea corresponds to the kingdom Archaebacteria.
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Domain Eukarya
The domain Eukarya consists of organisms that have a nucleus.
This domain is organized into four kingdoms:
Protista
Fungi
Plantae
Animalia
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16. The “Protists”: Unicellular Eukaryotes
The kingdom Protista has long been viewed by biologists as a “catchall” group of eukaryotes that could not be classified as fungi, plants, or animals.
Recent molecular studies and cladistic analyses have shown that the eukaryotes formerly known as “Protista” do not form a single clade. Current cladistic analysis divides these organisms into at least five clades.
Since these organisms cannot be properly placed into a single taxon, we refer to them as “protists.”

17. The “Protists”: Unicellular Eukaryotes
Most “protists” are unicellular, but one group, the brown algae, is multicellular.
Some “protists” are photosynthetic, while others are heterotrophic.
Some display characters that resemble those of fungi, plants, or animals.

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Fungi 
Members of the kingdom Fungi are heterotrophs with cell walls containing chitin.
Most fungi feed on dead or decaying organic matter by secreting digestive enzymes into it and absorbing small food molecules into their bodies.
They can be either multicellular (mushrooms) or unicellular (yeasts).
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Plantae
Members of the kingdom Plantae are multicellular, photosynthetic autotrophs.
Plants are nonmotile—they cannot move from place to place.
Plants have cell walls that contain cellulose.
The entire plant kingdom is the sister group to the red algae, which are “protists.” The plant kingdom, therefore, includes the green algae along with mosses, ferns, cone-bearing plants, and flowering plants.
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Animalia
Members of the kingdom Animalia are multicellular and heterotrophic.
The cells of animals do not have cell walls.
Most animals can move about, at least for some part of their life cycle.
There is great diversity within the animal kingdom, and many species exist in nearly every part of the planet.
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